Vapor Pressure and Heat of Vaporization
When a sample of a liquid is introduced into a container, the liquid will tend to evaporate.
Molecules will escape from the relative confinement of the liquid state into the gaseous
state. If the container is closed, this conversion will appear to stop when equilibrium is
achieved. Under equilibrium conditions, the rate of evaporation equals the rate of
condensation.
Liquid
Gas
A measure of the extent of vaporization is the vapor pressure—the partial pressure
exerted by the gas phase in equilibrium with the liquid phase. Other gases, e.g. air, can
be present, but what matters here is the pressure of the substance involved in the gasliquid equilibrium. Vapor pressure can be expressed in any convenient unit—mm Hg
(torr), atmospheres, bars, pascals, kilopascals, etc. The higher the vapor pressure of a
substance, the greater the concentration of the compound in the gaseous phase and the
greater the extent of vaporization. Liquids vary considerably in their vapor pressures. If
two substances are compared at the same temperature, the more volatile one will have the
higher vapor pressure.
You know from experience that increasing the temperature can increase the extent of
evaporation of a liquid. Indeed, vapor pressure increases with increasing temperature.
When the vapor pressure equals the atmospheric pressure, bubbles of vapor form and
escape. This temperature is defined as the normal boiling point.
In this experiment you will measure the vapor pressure of a liquid at a variety of
temperatures. You will find that a plot of the vapor pressure (p) versus temperature (T) is
not a straight line. The vapor pressure increases more rapidly than the temperature.
However, it is possible to obtain a straight line by making use of equation (1).
ln p = - ∆Hvap/RT + C
(1)
In equation (1), ln p indicates the natural logarithm (base e) of the vapor pressure, ∆Hvap
is the heat of vaporization of the compound, R is the gas constant (8.314 J/mole K), and T
is the equilibrium temperature (in Kelvin) corresponding to vapor pressure p. You don’t
need to worry about C, a constant of integration. Note that the variables you will be
measuring in this experiment are p and T. Equation (1) should yield a straight line if ln p
is plotted along the vertical (y) axis and 1/T is plotted along the horizontal (x) axis. The
slope of this line, d(ln p)/d(1/T) equals -∆Hvap/R. Thus, if a numerical value of the slope
is obtained from the graph of your experimental data, you should be able to calculate a
value for ∆Hvap. The heat of vaporization represents the amount of thermal energy
necessary to convert one mole of the liquid in question into the gaseous state. It is
usually reported in kJ/mol.
Advance Study Assignment
1.
The vapor pressure of acetone at 20oC is 185.5 mm Hg, and the vapor pressure of
water at this same temperature is 17.5 mm Hg. Predict which of the two liquids will
have a higher normal boiling point and explain why.
2.
Provide a molecular explanation that accounts for the differences in the vapor
pressures and boiling points of water and acetone.
3.
Sketch the appearance of a plot of ln p vs 1/T.
Experimental Procedure
In this experiment you will measure vapor pressure as a function of temperature, using a
digital gas pressure sensor interfaced with a Vernier LabPro and a TI graphing calculator.
The apparatus is pictured in Figure 1. A 125 mL suction flask is fitted with a two-hole
rubber stopper. The pressure sensor is attached via a plastic tube to a special fitting in
one of the holes in the stopper. Passing through the other hole in the stopper is one end
of a piece of straight glass tubing fitted with a Teflon stopcock. The glass tubing has an 8
mm outside diameter with the stopcock midway along its 30 cm length. This tube will be
used to introduce the liquid being studied into the flask.
Place a piece of clean filter paper into the clean
and dry flask. The purpose of the paper is to
absorb the liquid and facilitate the establishment
of equilibrium. Fit the rubber stopper tightly
into the flask and make sure the stopcock is
closed. Attach the side arm of the flask to an
aspirator, house vacuum port, vacuum line, or
vacuum pump using heavy walled rubber
tubing. Partially evacuate the flask until the
pressure sensor reads about 10 kPa. Close off
the rubber tubing on the side arm with a pinch
clamp. Place the flask in a beaker containing
ice and water. This will be your constant
temperature bath. Allow the apparatus to come
to thermal equilibrium. When the temperature
and pressure are constant (the former at or near
0oC) read and record the values of both. Use a
pipette to transfer approximately 2 mL of your
Figure 1: Experiment Apparatus
liquid to the sample-delivery tube. It is not
necessary to measure the exact amount (why
not?), but this volume will come close to filling the tube above the stopcock and will not
all vaporize when introduced into the flask. Introduce the liquid by opening the stopcock,
but close the stopcock before the liquid level drops below the stopcock and air is
introduced into the system.
Monitor the pressure; when it reaches a constant value, record that value and the
corresponding temperature. Without changing the flask or its connections, increase the
temperature of the water bath to about 10oC. When the temperature and pressure
stabilize, read and record both. Repeat temperature and pressure readings at roughly
20oC, 30oC, 40oC and 50oC. At this point you can disassemble the apparatus, remove the
filter paper, and clean and dry the flask. The apparatus is then ready for a repeat trial on
your original liquid or for a series of measurements on a new sample.
In processing your data, you need to take into consideration that you started the
experiment with some air in your flask. Hence, the pressure that you read after you
introduced your liquid is the sum of the pressure of the air originally present and the
vapor pressure of the sample. For a first approximation, assume that the pressure of the
air remains constant at all temperatures and is equal to the pressure measured at 0oC
before the introduction of the unknown. This assumption and your raw data will permit
you to calculate the vapor pressure for each temperature at which a measurement was
made. Enter your data in a spreadsheet and use the graphing program to plot vapor
pressure vs. absolute temperature. Then calculate ln p and 1/T for each set of
measurements and plot these two variables to examine to relationship specified in
equation (1), fitting the data with the best straight line. The equation of this line should
yield the slope, from which you can calculate a value for the heat of vaporization. The
equation also permits you to calculate the temperature at which the vapor pressure equals
1 atm, i.e. the normal boiling point.
On reflection, you should recognize that the method of treating the data described in the
previous paragraph makes an over simplification. The pressure of the air in the flask
does not remain constant as the temperature increases, even if the system doesn’t leak.
(Does it increase or decrease?) Your knowledge of the properties of ideal gases should
enable you to calculate the pressure of the air at each temperature, and hence obtain more
accurate values for the vapor pressure. It is your assignment to devise a strategy for
making this correction and to refine your calculation. Then use your new results to plot p
vs. T and ln p vs. 1/T, determine the equation of the straight line, and calculate the heat of
vaporization and the normal boiling point.
Your report should include your raw pressure and temperature data, sample calculations,
and an explanation of how you made the correction described in the previous paragraph.
Your spreadsheets and graphs are important parts of your report, as are your final results.
If you know the identity of your liquid, look up accepted values for the heat of
vaporization and normal boiling point and compare these results with your own. Discuss
the cause and nature of any errors.
ats 7/20/01